The effect of rare earth metals cerium, lanthanum and yttrium on chemical composition, structure and properties of electroless Co B alloy coating was studied. By plasma transmitting spectrograph, electron energy spectrometer, X ray diffractometter, micro hardometer and vibratory sample magnetometer the chemical constitution, structure and properties of the alloy coatings were analyzed and inspected. The results show that with a tiny quantity of rare earth metal added into Co B alloy coating, the content of boron is decreased in the alloy coatings, and the kinds of rare earth metal have enormous effect on the structure and properties of electroless Co B alloy coating. At the same time electroless Co B alloy with amorphous structure is transformed to electroless Co B RE alloy with microcrystalline or crystalline structure. In this way microhardness of the coatings is increased remarkably. Cerium and lanthanum would also increase the saturated magnetic intensity and decrease coercitive force of the coating. So soft magnetization of the coatings would be improved. The effect of rare earth metals cerium, lanthanum and yttrium on chemical composition, structure and properties of electroless Co B alloy coating was studied. By plasma transmitting spectrograph, electron energy spectrometer, X ray diffractometter, micro hardometer and vibratory sample magnetometer the chemical constitution, structure and properties of the alloy coatings were analyzed and inspected. The results show that with a tiny quantity of rare earth metal added into Co B alloy coating, the content of boron is decreased in the alloy coatings, and the kinds of rare earth metal have enormous effect on the structure and properties of electroless Co B alloy coating. At the same time electroless Co B alloy with amorphous structure is transformed to electroless Co B RE alloy with microcrystalline or crystalline structure. In this way microhardness of the coatings is increased remarkably. Cerium and lanthanum would also increase the saturated magnetic intensity and decrease coerc itive force of the coating. So soft magnetization of the coatings would be improved.